Mono- and diphenolase activity from fruit of Malus pumila |
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Authors: | Peter W Goodenough Sharon Kessell Andrew GH Lea T Loeffler |
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Institution: | Biochemistry Department, University of Bath, Claverton Down, Bath BA2 7AY, U.K. |
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Abstract: | Apple fruit used for beverage production has a polyphenol oxidase which does not hydroxylate tyrosine under any conditions but it hydroxylates p-coumaric acid in the presence of NADH, and phloridzin in the absence of cofactors. The apparent Kms for hydroxylation of phloridzin and p-coumaric acid are 1.5 and 4 mM, respectively. However, subsequent oxidation of 3-hydroxyphloridzin or caffeic acid has an apparent Km of 200 nM. The oxidation products of 3-hydroxyphloridzin are complex and a stable dimeric quinone is finally formed. The apparent Kms for oxidation of catechin, epicatechin, chlorogenic acid, l-Dopa and 4-methyl catechol are 4.7, 5.7, 6.0, 2.7 and 3.2 mM, respectively. The Km for oxygen was 4.3 % although there was marked substrate inhibition by oxygen above 30 %. Polyphenol oxidase was stable at pH 3.5–4.5 with an optimum of 4.5. |
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Keywords: | Rosaceae apple fruit phenolic oxidation polyphenol oxidase enzymic colour formation |
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